The production of honeycomb articles, such as, for example, diesel particulate filters, often involves the application of ceramic cements (also referred to as pastes or sealants) onto pre-formed honeycomb bodies. These cement compositions may be applied to the honeycomb body, for example to form plugs for the cell channels of the honeycomb, to form artificial skin (also known as “after-applied skin”), or to bond several smaller honeycomb segments together to make a larger honeycomb body.
Plugging of diesel particulate filters to force exhaust gas to flow through porous walls has historically been accomplished using one of two composition types. The first composition type is typically very similar to the raw material of the honeycomb body. For example, for a cordierite honeycomb filter, the first composition may include clay and talc, and firing to a temperature sufficient to convert the raw materials into cordierite. In various applications, this first composition type provides favorable rheology for plugging, due to the presence of clay and talc, which are platy raw materials (that is, flat and plate-like structures found in soils) with high surface charge. The second composition type may consist of pre-reacted, ground cordierite powder and both organic and inorganic binders. Without the inclusion of organic binders, the second composition type does not exhibit sufficient plasticity. Thus, to achieve favorable rheology for plugging, the second composition type is typically mixed with organic binder(s) (included for rheology) and then fired to about 1000° C. in order to remove those organic components and react the inorganic binders (included for structural strength after the organic components are burned off).
The second composition type has been modified to provide methods that do not require a firing process as described above. However, the second composition type still presents some challenges for commercialization, particularly in view of the rheological behavior of the composition which has been found to be prone to syneresis (that is, liquid separation from solids). For example, when the second composition type is used as a plugging application, the target plug depths often cannot be achieved because batch liquids wick into the porous ceramic walls and the cement dries out to a stiff state within a few millimeters. Although attempts have been made to modify the undesired behavior, to date the success has been limited.
One such attempted solution to try to improve flow in a cement batch has been to decrease particle size. However, decreasing particle size has been found to increase the level of voids and dimples in the plug material, which are typically considered undesirable for the plugging application. Another attempted solution for improving plug depth has been to increase the amounts of methylcellulose and/or water in the cement composition. However, this approach has also been found to increase voids and dimples in the plug material. Thus, a need still exists to find a composition, and method of making a composition that will exhibit acceptable rheology for application to ceramic honeycomb bodies, such as for use as a plug composition, while reducing voids and dimples in the plug.